This invention relates to spectral polarisation separators (SPSs). A particular, though not necessarily exclusive, application for such an SPS is in an amplified optical transmission system, where it is employed to overcome a phenomenon to which it has been appreciated that long-haul optically amplified systems are subject, namely "polarisation hole-burning" in erbium-doped optical amplifiers used to overcome fibre transmission loss. Polarisation hole burning results from the preferential saturation of the gain due to a subset of erbium ions which are associated with the polarisation-state of the gain-saturating signal.
The consequence of this phenomenon is that the polarisation-state which is orthogonal to the signal can see greater gain than that of the signal polarisation state. Amplified spontaneous emission noise (ASE) in the orthogonal state can thus build up along the system more rapidly than ASE in the signal polarisation state. In addition to causing undesirable additional noise, the orthogonal ASE can build up to an extent whereby the signal power drops in order to conserve the total energy that propagates along the system. As the signal power drops, so the SNR of the system suffers, thereby causing increased errors in the received data stream. It is therefore attractive to reduce the effects of polarisation hole-burning in amplified systems in order to maintain a system with good signal-to-noise characteristics.
A paper by T S Bergano et al. entitled, "A two wavelength depolarised transmitter for improved transmission performance in long-haul EDFA systems", Proc. 1993 IEEE LEOS Meeting, San Jose, November 1993, discloses a method of overcoming polarisation hole burning and non-linear effects in a long-haul communication system by means of the use of a two wavelength transmitter. In this construction, two laser sources, separated by approximately 0.3 nm, are combined with a fibre coupler before being launched, in the same polarisation state, through a conventional optical modulator. Transmission of the light from the two sources through this modulator, with the same polarisation state rather than with orthogonally related polarisation states, overcomes the polarisation sensitivity of a typical modulator, such as a device based on lithium-niobate. Then an SPS (spectral polarisation separator) is required to convert the co-polarised signals at the two wavelengths into substantially orthogonally polarised signals, such substantial orthogonal polarisation being a requirement for transmission down the amplified line in order to overcome the polarisation hole-burning effect in that line. A major improvement of 4.5 dB in the Q value of the system was demonstrated, which directly translates to an improvement in the bit-error-rate of the system.
However, the particular type of SPS described in that paper is far from ideal for use in a practical transmission system since it comprised a single piece of high birefringence fibre located downstream of the modulator, where its presence would have the effect of adding polarisation mode-dispersion (PMD) to the data (in the particular quoted example it would have added 13.5 ps). In combination with the PMD of a typical long-haul cable, this will have the effect of inducing inter symbol interference (ISI) which causes a decrease in the Q, or an increase in the error-rate of the system.